Motor control apparatus and image forming apparatus with limiting coil current flowing through motor coil

ABSTRACT

The motor control apparatus includes: a setting unit configured to set a limit value of coil current flowing through a coil of a motor; a current supply unit configured to supply the motor with the coil current in a range not exceeding the limit value set by the setting unit; a detection unit configured to detect a current value of the coil current; and a comparison unit configured to compare an average value of the current value detected by the detection unit over a predetermined time period with a first threshold value. When the average value has exceeded the first threshold value, the setting unit updates the limit value in a decreasing manner.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a motor control technique.

Description of the Related Art

A brushless motor is used as the drive source of a rotating member in animage forming apparatus. Japanese Patent Laid-Open No. 2001-209276discloses a configuration that limits the motor operating current basedon a limit value.

Along with downsizing of image forming apparatuses in recent years,brushless motors (hereinafter simply referred to as “motors”) serving asthe drive source of rotating members in such image forming apparatusesare also required to be downsized. Here, an unexpected increase of motorload may result in a rise of coil temperature due to an increase ofcurrent flowing through the coil of the motor (hereinafter, coilcurrent). When the coil temperature eventually exceeds the insulationtemperature of the coil, there may occur a motor failure. For example,using a motor with a small margin relative to the required output todownsize the motor makes the coil temperature more likely to exceed theinsulation temperature of the coil in case of an unexpected increase ofmotor load, whereby a motor failure may occur more frequently. However,excessively limiting the coil current in order to prevent motor failuremay hinder proper handling of load variation under normal operation.

SUMMARY OF THE INVENTION

According to the disclosure, a motor control apparatus includes: asetting unit configured to set a limit value of coil current flowingthrough a coil of a motor; a current supply unit configured to supplythe motor with the coil current in a range not exceeding the limit valueset by the setting unit; a detection unit configured to detect a currentvalue of the coil current; and a comparison unit configured to comparean average value of the current value detected by the detection unitover a predetermined time period with a first threshold value, wherein,when the average value has exceeded the first threshold value, thesetting unit updates the limit value in a decreasing manner.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a configuration diagram of an image forming apparatusaccording to one embodiment;

FIG. 2 is a configuration diagram of a motor control unit according toone embodiment;

FIG. 3 is a configuration diagram of a motor according to oneembodiment;

FIG. 4A illustrates a relation between the load torque and the coilcurrent;

FIG. 4B illustrates relations of the load torque with the coiltemperature and the switching element temperature, respectively.

FIG. 5 illustrates a variation of coil current due to the load variationunder normal load;

FIGS. 6A and 6B are explanatory diagrams of motor control according toone embodiment;

FIG. 7 is a flowchart of motor control according to one embodiment;

FIGS. 8A and 8B are explanatory diagrams of motor control according toone embodiment; and

FIG. 9 is a flowchart of motor control according to one embodiment.

DESCRIPTION OF THE EMBODIMENTS

Hereinafter, embodiments will be described in detail with reference tothe attached drawings. Note, the following embodiments are not intendedto limit the scope of the claimed invention. Multiple features aredescribed in the embodiments, but limitation is not made an inventionthat requires all such features, and multiple such features may becombined as appropriate. Furthermore, in the attached drawings, the samereference numerals are given to the same or similar configurations, andredundant description thereof is omitted.

First Embodiment

FIG. 1 is a configuration diagram of an image forming apparatusaccording to the present embodiment. The image forming apparatus forms afull color image by superimposing toner images including four colors:yellow (Y), magenta (M), cyan (C), and black (K). In FIG. 1 , Y, M, C,and K at ends of reference numerals indicate that the colors of thetoner images involved in the formation of members indicated by thereference numerals are yellow, magenta, cyan, and black. In thefollowing description, when it is not necessary to distinguish thecolors from each other, reference numerals excluding Y, M, C, and K atthe ends are used. During image formation, a photoconductor 13 isrotationally driven in the clockwise direction on the diagram. A chargeroller 15 charges the surface of the corresponding photoconductor 13 toa uniform electric potential. An exposing unit 11 exposes the surface ofthe corresponding photoconductor 13 to light to form an electrostaticlatent image on the photoconductor 13. A developing roller 16 of adeveloping unit 12 develops the electrostatic latent image on thecorresponding photoconductor 13 with toner and visualizes theelectrostatic latent image as a toner image. A primary transfer roller18 transfers, by a primary transfer bias, the toner image formed on thecorresponding photoconductor 13 to an intermediate transfer belt 19. Acleaner 14 removes the toner that is not transferred to the intermediatetransfer belt 19 and remaining on the corresponding photoconductor 13.Here, a full-color image is formed on the intermediate transfer belt 19by transferring toner images formed on each of the photoconductors 13 tothe intermediate transfer belt 19 in a superimposed manner.

The intermediate transfer belt 19 is rotationally driven in thecounter-clockwise direction on the diagram during image formation. Thetoner image transferred to the intermediate transfer belt 19 is therebyconveyed to an opposing position against a secondary transfer roller 29.On the other hand, a sheet 21 stacked on a cassette 22 is fed to aconveyance path from the cassette 22, and conveyed to the opposingposition against the secondary transfer roller 29 by rotation of eachroller provided along the conveyance path. The secondary transfer roller29 transfers, by a secondary transfer bias, the toner image on theintermediate transfer belt 19 to the sheet 21. Subsequently, the sheet21 is conveyed to a fixing unit 30. The fixing unit 30 heats andpressurizes the sheet 21 to fix the toner image to the sheet 21. Thesheet 21 on which the toner image has been fixed is discharged out ofthe image forming apparatus. A control unit 31 conducting overallcontrol of the image forming apparatus includes a CPU 32.

In the present embodiment, photoconductors 13Y, 13M and 13C arerotationally driven by a single motor. In addition, the photoconductor13K and the intermediate transfer belt 19 are rotationally driven by asingle motor. Furthermore, developing rollers 16Y, 16M, 16C and 16K arerotationally driven by a single motor. The control configurations ofthese motors are similar and will be described below, referring to FIG.2 .

FIG. 2 is a control configuration diagram of a motor 101. A motorcontrol unit 120 includes a microcomputer 121. A communication port 122of the microcomputer 121 performs serial communication with the controlunit 31. The control unit 31 controls rotation of the motor 101 bycontrolling the motor control unit 10 with serial communication. Areference clock generator 125 generates a reference clock based onoutput of a quartz oscillator 126. A counter 123 performs measurement orthe like of the pulse period, based on the reference clock. Anon-volatile memory 124 stores various types of data to be used formotor control, or programs to be executed by the microcomputer 121. Themicrocomputer 121 outputs a pulse width modulation signal (PWM signal)from a PWM port 127. In the present embodiment, the microcomputer 121outputs, for each of three phases (U, V, W) of the motor 101, a total ofsix PWM signals, namely, high-side PWM signals (U-H, V-H, W-H) andlow-side PWM signals (U-L, V-L, W-L). Accordingly, the PWM port 127includes six terminals, namely, U-H, V-H, W-H, U-L, V-L, and W-L.

Each terminal of the PWM port 127 is connected to a gate driver 132, andthe gate driver 132 performs on/off control of each switching element ofa three-phase inverter 131, based on the PWM signals. Note that theinverter 131 includes a total of six switching elements, i.e., three onthe high side and three on the low side, for each phase, and the gatedriver 132 controls each switching element based on a corresponding PWMsignal. A transistor or FET, for example, can be used as the switchingelement. It is assumed in the present embodiment that a high PWM signalturns ON the corresponding switching element, and a low PWM signal turnsOFF the corresponding switching element. An output 133 of the inverter131 is connected to coils 135 (U-phase), 136 (V-phase) and 137 (W-phase)of the motor 101. Performing ON/OFF control of each switching element ofthe inverter 131 allows for controlling the excitation current (coilcurrent) of the coils 135, 136 and 137, respectively. As has beendescribed above, the microcomputer 121, the gate driver 132, and theinverter 131 function as a current supply unit configured to supply coilcurrent to the plurality of coils 135, 136 and 137, and also control thecurrent value of the coil current.

A current sensor 130 outputs a detection voltage according to thecurrent value of the coil current flowing through each of the coils 135,136 and 137. An amplification unit 134 amplifies the detection voltageof each phase, applies an offset voltage thereto, and outputs theresulting voltage to an analog-to-digital converter (AD converter) 129.The AD converter 129 converts the detection voltage after amplificationinto a digital value. A current value calculation unit 128 determinesthe coil current of each phase based on an output value (digital value)of the AD converter 129. For example, it is assumed that the currentsensor 130 outputs a voltage of 0.01 V per 1 A, the amplification unit134 has an amplification factor (gain) of 10, and the offset voltageapplied by the amplifier 134 is 1.6 V. Assuming that the coil currentflowing through the motor 101 lies within a range of −10 A to +10 A, thevoltage output from the amplifier 134 turns out to be in a range of 0.6V to 2.6 V. For example, assuming that the AD converter 129 converts avoltage of 0 to 3 V into a digital value of 0 to 4095, a coil current of−10 A to +10 A is converted into a digital value approximately in arange of 819 to 3549. Here, it is assumed that the current value ispositive when the coil current flows from the inverter 131 toward themotor 101, otherwise the current value is negative.

The current value calculation unit 128 obtains the current value of thecoil current by subtracting an offset value corresponding to the offsetvoltage from the digital value, and multiplying the result with apredetermined conversion factor. In the present example, the offsetvalue corresponding to the offset voltage (1.6 V) is approximately 2184(1.6×4095/3). In addition, the conversion factor is approximately0.000733 (3/4095). As has been described above, the current sensor 130,the amplification unit 134, the AD converter 129, and the current valuecalculation unit 128 form a current detection unit that detects thecurrent value of the coil current.

FIG. 3 is a configuration diagram of the motor 101. The motor 101includes a 6-slot stator 140 and a four-pole rotor 141, the stator 140including the U-phase, V-phase, and W-phase coils 135, 136 and 137. Therotor 141, which is constituted by a permanent magnet, includes two setsof N-poles/S-poles.

FIG. 4A illustrates a relation between the load torque on the rotationaxis of the motor 101 and the coil current for rotating the rotor 141 ata predetermined target speed. As illustrated in FIG. 4A, the coilcurrent and load torque are in a proportional relation as represented bythe following Formula (1):Ic=(1/Kt)×T  (1)

In Formula (1), Ic is the coil current, T is the load torque, and Kt isthe torque constant of the motor 101. As is apparent from FIG. 4A andFormula (1), the larger the load, the larger the coil current.

FIG. 4B illustrates relations of the load torque applied on the rotationaxis of the motor 101 for rotating the rotor 141 at the samepredetermined target speed as in FIG. 4A, with the temperature of thecoil of the motor 101 and the temperature of the switching element ofthe inverter 131, respectively. The relation between the load torque andthe coil temperature can be expressed by the following Formula (2), andthe relation between the load torque and the switching elementtemperature can be expressed by the following Formula (3):Tc=a×T ²  (2)TF=b×T ²  (3)

In Formulae (2) and (3), Tc is the coil temperature, Tf is the switchingelement temperature, T is the load torque, a is the coil temperaturerise factor, and b is the temperature rise factor of the switchingelement. As is apparent from FIG. 4B and Formulae (2) and (3), increaseof the load torque causes rise of the coil temperature and switchingelement temperature.

For example, in a case where the rated temperature of the coil of themotor 101 is 120 degrees, insulation coating of the coil may melt byheat when the coil temperature exceeds 120 degrees, which may lead tofailure of the motor 101. Referring to FIG. 4B, the load torque is 80mNm when the coil temperature is 120 degrees. Referring to FIG. 4A, thecoil current value is 3.5 A when the load torque is 80 mNm. Therefore,it is basically required to set the coil current to 3.5 A or lower inorder to keep the coil temperature at 120 degrees or lower. Note that ina case that the coil current temporarily exceeds 3.5 A for a short timeperiod, if the coil temperature does not reach 120 degrees, theinsulating film will not melt. Further in a case where the coiltemperature has reached 120 degrees, if the duration is short, theinsulating film will not melt. Thus, it is necessary to prevent the coilcurrent from continuously exceeding 3.5 A for a predetermined timeperiod in order to prevent failure of the motor 101.

FIG. 5 illustrates an example of variation of the coil current over timewhile the motor 101 is rotating at a predetermined target speed under anormal state with no abnormality in the load of the motor 101. Even witha normal load, there may occur an instantaneous load variation. When theload has increased due to the load variation, the coil current increasesin order to prevent slowdown due to increase of load and maintain thetarget speed. In FIG. 5 , the coil current has exceeded 3.5 A twice.Here, limiting the coil current of the motor 101 to 3.5 A or lower maycause the rotation speed of the rotor 141 to decrease in the case ofsuch an increase of the load, which may result in an image defect suchas image shake, color shift, or the like. In order to maintain therotation speed of the rotor 141 at the target speed relative to loadvariation, it is necessary to be able to supply the coil currentrequired to cope with load variation. In other words, it is necessary tosupply the coil current so as to suppress variation of the rotationspeed of the rotor 141 under load variation, while preventing the coiltemperature from exceeding the rated temperature.

Therefore, in the present embodiment, a coil current limit value IL anda temperature rise threshold value Th are provided as parameters relatedto motor control. The coil current limit value IL is a variable value,whose initial value is set to a value that can cope with load variationwhile the rotor 141 is rotating at a target speed in a steady load state(under normal operation). For example, in a case where load variation inthe normal state is as illustrated in FIG. 5 , the maximum value of thecoil current under load variation turns out to be approximately 4 A.Therefore, the initial value of the coil current limit value IL isassumed to be 4 A or higher. In the following description, the initialvalue of the coil current limit value IL is assumed to be 5 A, which islarger, by 1 A, than the maximum value 4 A of the coil current underload variation. On the other hand, the temperature rise threshold valueTh is determined based on the current value of the coil current thatturns the coil temperature into the rated temperature. For example,assuming that characteristics of the motor 101 at the target speed arethose illustrated in FIGS. 4A and 4B, and the rated temperature of thecoil is 120 degrees, the current value of the coil current that turnsthe coil temperature into the rated temperature is 3.5 A. For example,the temperature rise threshold value Th can be set to 3.5 A.Alternatively, the temperature rise threshold value Th can be set to avalue taking into account a margin against 3.5 A. In the followingdescription, the temperature rise threshold value Th is assumed to be3.5 A. The control unit 31 obtains an average value for eachpredetermined time period of the coil current. Here, in the followingexample, the predetermined time period is assumed to be one second. Whenthe average value of the coil current for one second is equal to orlarger than the temperature rise threshold value Th, the control unit 31updates the coil current limit value IL in a stepwise manner by reducingit by a predetermined value. Here, in the present example, thepredetermined value is assumed to be 0.1 A.

FIG. 6A illustrates temporal variations of the coil current and the coilcurrent limit value IL under the normal load state. According to FIG.6A, although the coil current instantly exceeds the temperature risethreshold value Th (fixed to 3.5 A), the average value per second issmaller than the temperature rise threshold value Th and therefore thecoil current limit value IL remains at the initial value (5 A). FIG. 6Billustrates a temporal variation of the coil current and the coilcurrent limit value IL under an overloaded state. According to FIG. 6B,although the coil current is constantly flowing at about 4.3 A andinstantly rises, its maximum value is suppressed by the coil currentlimit value IL. In FIG. 6B, the average value of the coil current persecond is equal to or larger than the temperature rise threshold valueTh, and therefore the coil current limit value IL is updated in agradually decreasing manner (by 0.1 A at a time) from the initial value(5 A). In addition, when the coil current limit value IL falls below 4.3A, which is the current value of the constantly flowing coil current,the motor 101 can no longer maintain its speed and therefore stops.

As illustrated in FIG. 6A, the control according to the presentembodiment allows for coping with the load variation under normal load.On the other hand, as illustrated in FIG. 6B, it is possible to preventthe motor from failing due to an excessive rise of the coil temperatureunder overload. Here, in the present embodiment, the coil current limitvalue IL is assumed to be reduced by a predetermined value in a stepwisemanner regardless of the average value, in a case where the averagevalue of the coil current per second has exceeded the temperature risethreshold value Th. However, there may also be a configuration thatincreases the decrement value of the coil current limit value IL as theaverage value becomes larger, or the difference between the averagevalue and the temperature rise threshold value Th becomes larger.

FIG. 7 is a flowchart of a process to be performed by the control unit31 according to the present embodiment. Here, the control unit 31performs the process illustrated in FIG. 7 when the motor 101 startsrotating triggered by the start of image formation. When the motor 101reaches the target speed, the control unit 31 sets, at S10, the coilcurrent limit value IL to an initial value, which is 5 in the presentexample. Subsequently, the control unit 31 determines an average valuelave over a predetermined time period of coil current at S11, andcompares the average value lave with the temperature rise thresholdvalue Th at S12. When the average value lave is larger than thetemperature rise threshold value Th, the control unit 31 reduces, atS13, the coil current limit value IL by a predetermined value, which is0.1 in the present example. Subsequently, the control unit 31determines, at S14, whether or not image formation has been completed.In a case where the image formation has been completed, the control unit31 terminates the process of FIG. 7 . In a case where the imageformation has not been completed, the control unit 31 repeats theprocess from S11.

When, on the other hand, at S12, the average value lave is equal to orlower than the temperature rise threshold value Th, the control unit 31compares, at S15, the average value lave and a value (threshold value)obtained by subtracting a predetermined value from the temperature risethreshold value Th. Here, although the predetermined value is assumed tobe 1 in the present example, it is merely for illustrative purposes.When the average value lave is lower than the value obtained bysubtracting 1 from the temperature rise threshold value Th, the controlunit 31 increases, at S16, the coil current limit value IL by apredetermined value, which is 0.1 in the present example, and performsthe process of S14. When, on the other hand, the average value lave isequal to or larger than the value obtained by subtracting 1 from thetemperature rise threshold value Th, the control unit 31 performs theprocess of S14 without updating the coil current limit value IL. Here,the predetermined value used for reduction at S13 and the predeterminedvalue used for increase at S16 may be the same value or differentvalues. In addition, although it is assumed to increase the coil currentlimit value IL by a predetermined value at S16, there may also be aconfiguration that increases the increment value of the coil currentlimit value IL as the average value becomes smaller, or the differencebetween the average value and the value obtained by subtracting 1 fromthe temperature rise threshold value Th becomes larger.

As has been described above, dynamically controlling the coil currentlimit value IL based on the threshold value and the average value of thecoil current allows for preventing the coil temperature from exceedingthe rated temperature under overload (under abnormal load), while copingwith the load variation under normal operation.

Second Embodiment

Subsequently, there will be described a second embodiment, focusing ondifferences from the first embodiment. In the present embodiment, themotor speed is limited when the average value of the coil current islarger than the temperature rise threshold value Th. In the presentembodiment, a target speed limit value VL is further set, in addition tothe temperature rise threshold value Th and the coil current limit valueIL described in the first embodiment. The target speed limit value VL isa variable value, whose initial value is set to a value larger than theinitial target value of the rotation speed of the rotor 141(hereinafter, target speed initial value VTD). The target speed initialvalue VTD is a fixed value. Here, in the present embodiment, the coilcurrent limit value IL is a fixed value, unlikely to the firstembodiment. Similarly to the first embodiment, the initial value is setto a value (5 A in the present example) which allows for coping with theload variation when the rotor 141 is rotating at the target speedinitial value VTD in the steady load state (under normal operation). Thecontrol unit 31 then updates the target speed limit value VL in astepwise manner by reducing it by a predetermined value in a case wherethe average value of the coil current per second is equal to or largerthan the temperature rise threshold value Th. In the following example,the initial value of the target speed limit value VL is assumed to be2700 rpm, and the target speed initial value VTD is assumed to be 2000rpm. While the target speed limit value VL is equal to or larger thanthe target speed initial value VTD, the control unit 31 determines thetarget speed initial value VTD to be the target value VT of the rotationspeed of the rotor 141. When, on the other hand, the target speed limitvalue VL falls below the target speed initial value VTD, the controlunit 31 determines the target speed limit value VL to be the targetvalue VT.

FIG. 8A illustrates a temporal variation of the target speed limit valueVL under overload, and FIG. 8B illustrates a temporal variation of thecoil current corresponding to FIG. 8A. As illustrated in FIG. 8B, theaverage value of the coil current is equal to or larger than thetemperature rise threshold value Th, and therefore the target speedlimit value VL has been updated in a manner gradually decreasing fromthe initial value (2700 rpm). Here, the decrement value is set to 100rpm in the present example. When the target speed limit value VL reachesor falls below the target speed initial value VTD (2000 rpm), the targetvalue VT of the rotation speed of the rotor 141 is set to the targetspeed limit value VL. In other words, the rotation speed of the rotor141 is reduced from its initial value. In accordance with the decreaseof the rotation speed of the rotor 141, the coil current decreases. Whenthe rotation speed of the rotor 141 falls below the predetermined speed,the motor 101 can no longer continue its rotation and therefore stops.

As illustrated in FIGS. 8A and 8B, the control according to the presentembodiment allows for preventing the motor from failing due to anexcessive rise of the coil temperature under overload. Here, in thepresent embodiment, the target speed limit value VL is assumed to bereduced by a predetermined value in a stepwise manner regardless of theaverage value, in a case where the average value of the coil current persecond has exceeded the temperature rise threshold value Th. However,there may also be a configuration that increases the decrement value ofthe target speed limit value VL as the average value becomes larger, orthe difference between the average value and the temperature risethreshold value Th becomes larger.

FIG. 9 is a flowchart of a process to be performed by the control unit31 in the present embodiment. At S20, the control unit 31 sets thetarget current limit value VL to an initial value, which is 2700 in thepresent example, sets the target speed initial value VTD to 2000, androtates the rotor 141 so as to reach the target speed initial value VTD.Subsequently, the control unit 31 determines the average value lave ofcoil current at S21 over a predetermined time period, and compares, atS22, the average value lave with the temperature rise threshold valueTh. When the average value lave is larger than the temperature risethreshold value Th, the control unit 31 reduces, at S23, the targetspeed limit value VL by a predetermined value, which is 100 in thepresent example. Subsequently, at S24, the control unit 31 sets thesmaller one of VTD and VL to be the target value VT. The control unit 31then determines, at S25, whether the image formation has been completed.In a case where the image formation has been completed, the control unit31 terminates the process of FIG. 9 . In a case where the imageformation has not been completed, the control unit 31 repeats theprocess from S21.

When, on the other hand, the average value lave is equal to or lowerthan the temperature rise threshold value Th at S22, the control unit 31compares, at S26, the average value lave and the value (threshold value)obtained by subtracting a predetermined value from the temperature risethreshold value Th. Here, although the predetermined value is assumed tobe 1 in the present example, it is merely for illustrative purposes.When the average value lave is lower than the value obtained bysubtracting 1 from the temperature rise threshold value Th, the controlunit 31 increases, at S27, the target speed limit value VL by apredetermined value, which is 100 in the present example, and performsthe process of S24. When, on the other hand, the average value lave isequal to or larger than the value obtained by subtracting 1 from thetemperature rise threshold value Th, the control unit 31 performs theprocess of S24 without updating the target speed limit value VL. Here,the predetermined value used for reduction at S23 and the predeterminedvalue used for increase at S27 may be the same value or differentvalues. In addition, although it is assumed to increase the coil currentlimit value IL by a predetermined value at S27, there may also be aconfiguration that increases the increment value of the target speedlimit value VL as the average value becomes smaller, or the differencebetween the average value and the value obtained by subtracting 1 fromthe temperature rise threshold value Th becomes larger.

As has been described above, dynamically controlling the target value ofthe rotation speed of the rotor 141 based on the threshold value and theaverage value of the coil current allows for preventing the coiltemperature from exceeding the rated temperature under overload (underabnormal load), while coping with the load variation under normaloperation.

Other Embodiments

Note that there may be a configuration in which the motor control unit120 performs some or all of the processes assumed to be performed by thecontrol unit 31 in the aforementioned embodiment. In addition, the motorcontrol unit 120 and the motor-control-related part of the control unit31 can be implemented as a motor control apparatus. In addition,although the motor 101 is assumed to be the drive source of thephotoconductor 13, the intermediate transfer belt 19, and the developingroller 16, the load applied to the motor 101 may be a roller conveyingthe sheet 21, a fixing device 30, or the like, with no limitation on thetype of load. Furthermore, although the present embodiment has beendescribed as an image forming apparatus, the present invention can beapplied to any device that controls the motor 101. Furthermore, specificnumerical values used in the aforementioned embodiments are exemplary,and the present invention is not limited to specific numerical valuesused in the description of the embodiments.

Embodiments of the present invention can also be realized by a computerof a system or apparatus that reads out and executes computer executableinstructions (e.g., one or more programs) recorded on a storage medium(which may also be referred to more fully as a ‘non-transitorycomputer-readable storage medium’) to perform the functions of one ormore of the above-described embodiments and/or that includes one or morecircuits (e.g., application specific integrated circuit (ASIC)) forperforming the functions of one or more of the above-describedembodiments, and by a method performed by the computer of the system orapparatus by, for example, reading out and executing the computerexecutable instructions from the storage medium to perform the functionsof one or more of the above-described embodiments and/or controlling theone or more circuits to perform the functions of one or more of theabove-described embodiment(s). The computer may comprise one or moreprocessors (e.g., central processing unit (CPU), micro processing unit(MPU)) and may include a network of separate computers or separateprocessors to read out and execute the computer executable instructions.The computer executable instructions may be provided to the computer,for example, from a network or the storage medium. The storage mediummay include, for example, one or more of a hard disk, a random-accessmemory (RAM), a read only memory (ROM), a storage of distributedcomputing systems, an optical disk (such as a compact disc (CD), digitalversatile disc (DVD), or Blu-ray Disc (BD)™), a flash memory device, amemory card, and the like.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2019-215598, filed Nov. 28, 2019, which is hereby incorporated byreference herein in its entirety.

What is claimed is:
 1. A motor control apparatus comprising: a settingunit configured to set a limit value of coil current flowing through acoil of a motor; a current supply unit configured to supply the motorwith the coil current in a range not exceeding the limit value set bythe setting unit; a detection unit configured to detect a current valueof the coil current; and a comparison unit configured to compare anaverage value of the current value detected by the detection unit over apredetermined time period with a first threshold value, the firstthreshold value being smaller than the limit value, wherein, the firstthreshold value is determined based on the current value of the coilcurrent that turns a coil temperature into a rated temperature, and whenthe average value has exceeded the first threshold value, the settingunit updates the limit value in a decreasing manner.
 2. The motorcontrol apparatus according to claim 1, wherein the current supply unitsupplies the motor with the coil current for rotating a rotor of themotor at a predetermined target speed, and an initial value of the limitvalue set by the setting unit is larger than a value of the coil currentrequired to rotate the rotor at the predetermined target speed relativeto load variation of the motor.
 3. The motor control apparatus accordingto claim 1, wherein, when the average value has exceeded the firstthreshold value, the setting unit updates the limit value in adecreasing manner by a first predetermined value.
 4. The motor controlapparatus according to claim 1, wherein, when the average value hasexceeded the first threshold value, the setting unit updates the limitvalue in a decreasing manner by a decrement value obtained based ondifference between the average value and the first threshold value. 5.The motor control apparatus according to claim 4, wherein the settingunit increases the decrement value, as the difference between theaverage value and the first threshold value becomes larger.
 6. The motorcontrol apparatus according to claim 1, wherein, when the average valueis lower than a second threshold value, the setting unit updates thelimit value in an increasing manner, and the second threshold value islower than the first threshold value.
 7. The motor control apparatusaccording to claim 6, wherein, when the average value is lower than thesecond threshold value, the setting unit updates the limit value in anincreasing manner by a second predetermined value.
 8. The motor controlapparatus according to claim 6, wherein, when the average value is lowerthan the second threshold value, the setting unit updates the limitvalue in an increasing manner by an increment value obtained based ondifference between the average value and the second threshold value. 9.The motor control apparatus according to claim 8, wherein the settingunit increases the increment value, as the difference between theaverage value and the second threshold value becomes larger.
 10. A motorcontrol apparatus comprising: a setting unit configured to set a limitvalue of rotation speed of a rotor of a motor; a determination unitconfigured to determine a target value of a rotation speed of the rotorbased on a target speed initial value and the limit value; a currentsupply unit configured to supply a coil of the motor with coil currentfor rotating the rotor at the target value; a detection unit configuredto detect a current value of the coil current; and a comparison unitconfigured to compare an average value of the current value detected bythe detection unit over a predetermined time period with a firstthreshold value, the first threshold value being determined based on thecurrent value of the coil current that turns a coil temperature into arated temperature, wherein, when the average value has exceeded thefirst threshold value, the setting unit updates the limit value in adecreasing manner, and the determination unit determines a smaller oneof the limit value and the target speed initial value to be the targetvalue.
 11. The motor control apparatus according to claim 10, whereinthe target speed initial value is lower than an initial value of thelimit value.
 12. The motor control apparatus according to claim 10,wherein the current supply unit supplies the motor with the coil currentin a range not exceeding a predetermined maximum value and the maximumvalue is larger than a value of the coil current required to rotate therotor at the target initial speed value relative to load variation ofthe motor.
 13. The motor control apparatus according to claim 10,wherein, when the average value has exceeded the first threshold value,the setting unit updates the limit value in a decreasing manner by afirst predetermined value.
 14. The motor control apparatus according toclaim 10, wherein, when the average value has exceeded the firstthreshold value, the setting unit updates the limit value in adecreasing manner by a decrement value obtained based on differencebetween the average value and the first threshold value.
 15. The motorcontrol apparatus according to claim 14, wherein the setting unitincreases the decrement value, as the difference between the averagevalue and the first threshold value becomes larger.
 16. The motorcontrol apparatus according to claim 10, wherein, when the average valueis lower than a second threshold value, the setting unit updates thelimit value in an increasing manner, and the second threshold value islower than the first threshold value.
 17. The motor control apparatusaccording to claim 16, wherein, when the average value is lower than thesecond threshold value, the setting unit updates the limit value in anincreasing manner by a second predetermined value.
 18. The motor controlapparatus according to claim 16, wherein, when the average value islower than the second threshold value, the setting unit updates thelimit value in an increasing manner by an increment value obtained basedon difference between the average value and the second threshold value.19. The motor control apparatus according to claim 18, wherein thesetting unit increases the increment value, as the difference betweenthe average value and the second threshold value becomes larger.
 20. Animage forming apparatus comprising: an image forming unit configured toform an image on a sheet; a motor for rotationally driving a rotatingmember of the image forming unit; a setting unit configured to set alimit value of coil current flowing through a coil of the motor; acurrent supply unit configured to supply the motor with the coil currentin a range not exceeding the limit value set by the setting unit; adetection unit configured to detect a current value of the coil current;and a comparison unit configured to compare an average value of thecurrent value detected by the detection unit over a predetermined timeperiod with a first threshold value, the first value being smaller thanthe limit value, wherein, the first threshold value is determined basedon the current value of the coil current that turns a coil temperatureinto a rated temperature, and when the average value has exceeded thefirst threshold value, the setting unit updates the limit value in adecreasing manner.
 21. An image forming apparatus comprising: an imageforming unit configured to form an image on a sheet; a motor forrotationally driving a rotating member of the image forming unit; asetting unit configured to set a limit value of a rotation speed of arotor of the motor; a determination unit configured to determine atarget value of rotation speed of the rotor based on a target speedinitial value and the limit value; a current supply unit configured tosupply a coil of the motor with coil current to rotate the rotor at thetarget value; a detection unit configured to detect a current value ofthe coil current; and a comparison unit configured to compare an averagevalue of the current value detected by the detection unit over apredetermined time period with a first threshold value, the firstthreshold value being determined based on the current value of the coilcurrent that turns a coil temperature into a rated temperature, wherein,when the average value has exceeded the first threshold value, thesetting unit updates the limit value in a decreasing manner, and thedetermination unit determines a smaller one of the limit value and thetarget speed initial value to be the target value.